1. Field of the Invention
The present invention relates to a curable resin composition useful for encapsulation or the like of optical devices such as light-emitting devices.
2. Description of the Related Art
Traditionally, bisphenol A glycidyl ether type epoxy resins are used for encapsulating LEDs. However, such resins exhibit inferior heat resistance and light resistance (in particular, with regard to resistance to both UV and blue light). Hence, when these resins are used in high-intensity LEDs, UV LEDs, or the like, the resins are discolored by the heat and light emitted from the LEDs.
Therefore, a problem exists in that the luminance of LEDs varies over time.
In response to this problem, highly transparent epoxy resins have been developed. However, the heat resistance and light resistance of such resins are still not satisfactory.
In view of the above, gel-type silicone resins have been used in high-intensity LEDs as they exhibit excellent heat resistance and light resistance when compared with epoxy resins.
However, gel-type silicone resins have the following problems.
First, the surface of these silicone resins is sticky, so that dust and dirt easily adhere thereto. Therefore, at present, the application of these silicone resins is limited to use as a resin for filling the gap formed after the dome portion of an LED, where the dome portion functions as a lens, is joined to the base of an LED chip, and to use as an encapsulating resin when an LED is surface mounted.
Second, since the refractive index of the silicone resins used falls within the range of 1.41 to 1.51 and is lower than that of epoxy resins, the silicone resins reduce the light extraction efficiency of the LEDs that include such. Specifically, in high intensity LEDs, a sapphire substrate is often used as the chip substrate thereof, and a method in which the light is extracted from the sapphire substrate side of the high intensity LEDs is usually therefore employed. The refractive index of sapphire is 1.76. Therefore, in order to efficiently extract light from the sapphire substrate into an encapsulating resin, it is preferable that the refractive index of the encapsulating resin be close to the refractive index of sapphire, i.e., 1.76. However, among the silicone resins generally used, dimethyl silicone resin has a refractive index of 1.41. Furthermore, diphenyl dimethyl-based and phenyl methyl-based silicone resins, into which a phenyl group is introduced to increase the refractive index thereof, have a refractive index of approximately 1.51. Therefore, the refractive index of such silicone resins is lower than that of epoxy resins which have a refractive index in the range of 1.53 to 1.57. Therefore, when a silicone resin is used as the encapsulating resin for high-intensity LEDs, it is inevitable that the light extraction efficiency of such is lower than that which is achieved when an epoxy resin is used.
Third, since the silicone resins used in electronic materials are of an addition reaction type and are two-part resins, the two parts are required to be mixed immediately prior to use. Generally, the two parts are mixed using a static mixer. However, this mixer can mix only relatively low viscosity materials, and therefore it is difficult to obtain a resin composition having a sufficiently high viscosity after the mixing of the two parts. Hence, such resins cannot be molded into a predetermined lens shape, and a lens function cannot be imparted to the encapsulating resins.
Among the problems in association with the silicone resins, the problem of their refractive index may be solved by a technique proposed in Japanese Patent Application Laid-Open No. 2004-15063. Specifically, in this technique, the refractive index of a resin composition is increased by adding fine particles of titanium oxide, zirconium oxide, zinc oxide, or the like having a high refractive index to the resin. However, in order to increase the refractive index of the silicone resins to a level that is greater than that of epoxy resins by using this technique, at least 10 to 40% by volume of the fine particles must be added thereto. Unfortunately, the addition of these fine particles tends to reduce the transparency of the silicone resins. Moreover, it is difficult to obtain the level of fluidity necessary to enable the silicone resins to be used as an encapsulating resin. Furthermore, there has also been an attempt to improve the transparency of silicone resins using fine particles called single-nano particles. However, the cohesive force of ultrafine particles of single-nano size is very strong, and therefore it is very difficult to uniformly disperse the ultrafine particles within a resin without forming secondary aggregated particles. Therefore, a technology for encapsulating LEDs in a resin containing such fine particles has not yet been practically realized.
Meanwhile, the use of a fluorene group-containing monoacrylate as a high-refractive index resin used in the manufacture of an optical antireflective film has been proposed in Japanese Patent Application Laid-Open No. 2002-293762. This compound may also be considered for use as an encapsulating resin for LEDs.
However, since fluorene group-containing monoacrylates have very high viscosity, its handleability as an encapsulating agent is poor. When a low-viscosity diluent such as 2-hydroxyethyl acrylate is added to a fluorene group-containing monoacrylate, the viscosity of the composition can be reduced. However, a problem arises in that the refractive index of the final resin is also reduced by use of the diluent. Furthermore, this composition becomes very hard once the curing process is completed. Hence, when the composition is used as an encapsulating resin for LEDs, thermal stress may cause problems such as peeling of the resin away from the LED chip, breakage of the chip, breakages in the wiring, and the like.